diff --git a/uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/models/Habit.kt b/uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/models/Habit.kt
index 132250754..07be68a00 100644
--- a/uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/models/Habit.kt
+++ b/uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/models/Habit.kt
@@ -107,6 +107,10 @@ data class Habit(
         )
     }
 
+    fun firstEntryDate(): Timestamp {
+        return computedEntries.getKnown().lastOrNull()?.timestamp ?: DateUtils.getTodayWithOffset()
+    }
+
     fun copyFrom(other: Habit) {
         this.color = other.color
         this.description = other.description
diff --git a/uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/models/HabitGroup.kt b/uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/models/HabitGroup.kt
new file mode 100644
index 000000000..6d8773732
--- /dev/null
+++ b/uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/models/HabitGroup.kt
@@ -0,0 +1,123 @@
+package org.isoron.uhabits.core.models
+
+import org.isoron.uhabits.core.utils.DateUtils
+import java.util.UUID
+
+data class HabitGroup(
+    var color: PaletteColor = PaletteColor(8),
+    var description: String = "",
+    var id: Long? = null,
+    var isArchived: Boolean = false,
+    var name: String = "",
+    var position: Int = 0,
+    var question: String = "",
+    var reminder: Reminder? = null,
+    var unit: String = "",
+    var uuid: String? = null,
+    var habitList: HabitList,
+    var habitGroupList: HabitGroupList,
+    val scores: ScoreList,
+    val streaks: StreakList
+) {
+    init {
+        if (uuid == null) this.uuid = UUID.randomUUID().toString().replace("-", "")
+    }
+
+    var observable = ModelObservable()
+
+    val uriString: String
+        get() = "content://org.isoron.uhabits/habit/$id"
+
+    fun hasReminder(): Boolean = reminder != null
+
+    fun isCompletedToday(): Boolean {
+        return habitList.all { it.isCompletedToday() } && habitGroupList.all { it.isCompletedToday() }
+    }
+
+    fun isEnteredToday(): Boolean {
+        return habitList.all { it.isEnteredToday() } && habitGroupList.all { it.isEnteredToday() }
+    }
+
+    fun firstEntryDate(): Timestamp {
+        val today = DateUtils.getTodayWithOffset()
+        var earliest = today
+        for (h in habitList) {
+            val first = h.firstEntryDate()
+            if (earliest.isNewerThan(first)) earliest = first
+        }
+        for (hgr in habitGroupList) {
+            val first = hgr.firstEntryDate()
+            if (earliest.isNewerThan(first)) earliest = first
+        }
+        return earliest
+    }
+
+    fun recompute() {
+        for (h in habitList) h.recompute()
+        for (hgr in habitGroupList) hgr.recompute()
+
+        val today = DateUtils.getTodayWithOffset()
+        val to = today.plus(30)
+        var from = firstEntryDate()
+        if (from.isNewerThan(to)) from = to
+
+        scores.combineFrom(
+            habitList = habitList,
+            habitGroupList = habitGroupList,
+            from = from,
+            to = to
+        )
+
+        streaks.combineFrom(
+            habitList = habitList,
+            habitGroupList = habitGroupList,
+            from = from,
+            to = to
+        )
+    }
+
+    fun copyFrom(other: Habit) {
+        this.color = other.color
+        this.description = other.description
+        // this.id should not be copied
+        this.isArchived = other.isArchived
+        this.name = other.name
+        this.position = other.position
+        this.question = other.question
+        this.reminder = other.reminder
+        this.unit = other.unit
+        this.uuid = other.uuid
+    }
+
+    override fun equals(other: Any?): Boolean {
+        if (this === other) return true
+        if (other !is Habit) return false
+
+        if (color != other.color) return false
+        if (description != other.description) return false
+        if (id != other.id) return false
+        if (isArchived != other.isArchived) return false
+        if (name != other.name) return false
+        if (position != other.position) return false
+        if (question != other.question) return false
+        if (reminder != other.reminder) return false
+        if (unit != other.unit) return false
+        if (uuid != other.uuid) return false
+
+        return true
+    }
+
+    override fun hashCode(): Int {
+        var result = color.hashCode()
+        result = 31 * result + description.hashCode()
+        result = 31 * result + (id?.hashCode() ?: 0)
+        result = 31 * result + isArchived.hashCode()
+        result = 31 * result + name.hashCode()
+        result = 31 * result + position
+        result = 31 * result + question.hashCode()
+        result = 31 * result + (reminder?.hashCode() ?: 0)
+        result = 31 * result + unit.hashCode()
+        result = 31 * result + (uuid?.hashCode() ?: 0)
+        return result
+    }
+}
diff --git a/uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/models/HabitGroupList.kt b/uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/models/HabitGroupList.kt
new file mode 100644
index 000000000..583e5c82f
--- /dev/null
+++ b/uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/models/HabitGroupList.kt
@@ -0,0 +1,200 @@
+package org.isoron.uhabits.core.models
+
+import com.opencsv.CSVWriter
+import java.io.IOException
+import java.io.Writer
+import java.util.LinkedList
+import javax.annotation.concurrent.ThreadSafe
+
+/**
+ * An ordered collection of [HabitGroup]s.
+ */
+@ThreadSafe
+abstract class HabitGroupList : Iterable<HabitGroup> {
+    val observable: ModelObservable
+
+    @JvmField
+    protected val filter: HabitMatcher
+
+    /**
+     * Creates a new HabitList.
+     *
+     * Depending on the implementation, this list can either be empty or be
+     * populated by some pre-existing habits, for example, from a certain
+     * database.
+     */
+    constructor() {
+        observable = ModelObservable()
+        filter = HabitMatcher(isArchivedAllowed = true)
+    }
+
+    protected constructor(filter: HabitMatcher) {
+        observable = ModelObservable()
+        this.filter = filter
+    }
+
+    /**
+     * Inserts a new habit in the list.
+     *
+     * If the id of the habit is null, the list will assign it a new id, which
+     * is guaranteed to be unique in the scope of the list. If id is not null,
+     * the caller should make sure that the list does not already contain
+     * another habit with same id, otherwise a RuntimeException will be thrown.
+     *
+     * @param habitGroup the habit to be inserted
+     * @throws IllegalArgumentException if the habit is already on the list.
+     */
+    @Throws(IllegalArgumentException::class)
+    abstract fun add(habitGroup: HabitGroup)
+
+    /**
+     * Returns the habit with specified id.
+     *
+     * @param id the id of the habit
+     * @return the habit, or null if none exist
+     */
+    abstract fun getById(id: Long): HabitGroup?
+
+    /**
+     * Returns the habit with specified UUID.
+     *
+     * @param uuid the UUID of the habit
+     * @return the habit, or null if none exist
+     */
+    abstract fun getByUUID(uuid: String?): HabitGroup?
+
+    /**
+     * Returns the habit that occupies a certain position.
+     *
+     * @param position the position of the desired habit
+     * @return the habit at that position
+     * @throws IndexOutOfBoundsException when the position is invalid
+     */
+    abstract fun getByPosition(position: Int): HabitGroup
+
+    /**
+     * Returns the list of habits that match a given condition.
+     *
+     * @param matcher the matcher that checks the condition
+     * @return the list of matching habits
+     */
+    abstract fun getFiltered(matcher: HabitMatcher?): HabitGroupList
+    abstract var primaryOrder: Order
+    abstract var secondaryOrder: Order
+
+    /**
+     * Returns the index of the given habit in the list, or -1 if the list does
+     * not contain the habit.
+     *
+     * @param h the habit
+     * @return the index of the habit, or -1 if not in the list
+     */
+    abstract fun indexOf(h: HabitGroup): Int
+    val isEmpty: Boolean
+        get() = size() == 0
+
+    /**
+     * Removes the given habit from the list.
+     *
+     * If the given habit is not in the list, does nothing.
+     *
+     * @param h the habit to be removed.
+     */
+    abstract fun remove(h: HabitGroup)
+
+    /**
+     * Removes all the habits from the list.
+     */
+    open fun removeAll() {
+        val copy: MutableList<HabitGroup> = LinkedList()
+        for (h in this) copy.add(h)
+        for (h in copy) remove(h)
+        observable.notifyListeners()
+    }
+
+    /**
+     * Changes the position of a habit in the list.
+     *
+     * @param from the habit that should be moved
+     * @param to   the habit that currently occupies the desired position
+     */
+    abstract fun reorder(from: HabitGroup, to: HabitGroup)
+    open fun repair() {}
+
+    /**
+     * Returns the number of habits in this list.
+     *
+     * @return number of habits
+     */
+    abstract fun size(): Int
+
+    /**
+     * Notifies the list that a certain list of habits has been modified.
+     *
+     * Depending on the implementation, this operation might trigger a write to
+     * disk, or do nothing at all. To make sure that the habits get persisted,
+     * this operation must be called.
+     *
+     * @param habitGroups the list of habits that have been modified.
+     */
+    abstract fun update(habitGroups: List<HabitGroup>)
+
+    /**
+     * Notifies the list that a certain habit has been modified.
+     *
+     * See [.update] for more details.
+     *
+     * @param habitGroup the habit that has been modified.
+     */
+    fun update(habitGroup: HabitGroup) {
+        update(listOf(habitGroup))
+    }
+
+    /**
+     * Writes the list of habits to the given writer, in CSV format. There is
+     * one line for each habit, containing the fields name, description,
+     * frequency numerator, frequency denominator and color. The color is
+     * written in HTML format (#000000).
+     *
+     * @param out the writer that will receive the result
+     * @throws IOException if write operations fail
+     */
+    @Throws(IOException::class)
+    fun writeCSV(out: Writer) {
+        val header = arrayOf(
+            "Position",
+            "Name",
+            "Question",
+            "Description",
+            "NumRepetitions",
+            "Interval",
+            "Color"
+        )
+        val csv = CSVWriter(out)
+        csv.writeNext(header, false)
+        for (habit in this) {
+            val cols = arrayOf(
+                String.format("%03d", indexOf(habit) + 1),
+                habit.name,
+                habit.question,
+                habit.description,
+                habit.color.toCsvColor()
+            )
+            csv.writeNext(cols, false)
+        }
+        csv.close()
+    }
+
+    abstract fun resort()
+    enum class Order {
+        BY_NAME_ASC,
+        BY_NAME_DESC,
+        BY_COLOR_ASC,
+        BY_COLOR_DESC,
+        BY_SCORE_ASC,
+        BY_SCORE_DESC,
+        BY_STATUS_ASC,
+        BY_STATUS_DESC,
+        BY_POSITION
+    }
+}
diff --git a/uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/models/ScoreList.kt b/uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/models/ScoreList.kt
index 84945ad97..b608334b0 100644
--- a/uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/models/ScoreList.kt
+++ b/uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/models/ScoreList.kt
@@ -19,8 +19,6 @@
 package org.isoron.uhabits.core.models
 
 import org.isoron.uhabits.core.models.Score.Companion.compute
-import java.util.ArrayList
-import java.util.HashMap
 import javax.annotation.concurrent.ThreadSafe
 import kotlin.math.max
 import kotlin.math.min
@@ -139,4 +137,21 @@ class ScoreList {
             map[timestamp] = Score(timestamp, previousValue)
         }
     }
+
+    @Synchronized
+    fun combineFrom(
+        habitList: HabitList,
+        habitGroupList: HabitGroupList,
+        from: Timestamp,
+        to: Timestamp
+    ) {
+        var current = to
+        while (current >= from) {
+            val habitScores = habitList.map { it.scores[current].value }
+            val groupScores = habitGroupList.map { it.scores[current].value }
+            val averageScore = (habitScores + groupScores).average()
+            map[current] = Score(current, averageScore)
+            current = current.minus(1)
+        }
+    }
 }
diff --git a/uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/models/Streak.kt b/uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/models/Streak.kt
index 582dbf6f1..446f394b2 100644
--- a/uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/models/Streak.kt
+++ b/uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/models/Streak.kt
@@ -38,4 +38,8 @@ data class Streak(
 
     val length: Int
         get() = start.daysUntil(end) + 1
+
+    fun isInStreak(timestamp: Timestamp): Boolean {
+        return timestamp in start..end
+    }
 }
diff --git a/uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/models/StreakList.kt b/uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/models/StreakList.kt
index d3bed0af7..466e7b596 100644
--- a/uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/models/StreakList.kt
+++ b/uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/models/StreakList.kt
@@ -62,4 +62,35 @@ class StreakList {
         }
         list.add(Streak(begin, end))
     }
+
+    @Synchronized
+    fun isInStreaks(timestamp: Timestamp): Boolean {
+        return list.any { it.isInStreak(timestamp) }
+    }
+
+    @Synchronized
+    fun combineFrom(
+        habitList: HabitList,
+        habitGroupList: HabitGroupList,
+        from: Timestamp,
+        to: Timestamp
+    ) {
+        var current = from
+        var streakRunning = false
+        var streakStart = from
+        while (current <= to) {
+            if (habitList.all { it.streaks.isInStreaks(current) } &&
+                habitGroupList.all { it.streaks.isInStreaks(current) } &&
+                !streakRunning
+            ) {
+                streakStart = current
+                streakRunning = true
+            } else if (streakRunning) {
+                val streakEnd = current.minus(1)
+                list.add(Streak(streakStart, streakEnd))
+                streakRunning = false
+            }
+            current = current.plus(1)
+        }
+    }
 }